Hrmm, weird. Have you tried re-booting?
The only thing I would suspect is the sheet you are doing a vlookup on had the the row outside of what is expected, so if you press shft-ctrl-down on it it doesn't stop at your last data entry, it goes way below. You could test doing that outside a formula, just to see. Of course, you should be able to manually enter the formula but who wants to type all that, eh?